Code signal generator



Jan. 15, 1963 w. s. DRUz 3,073,892

CODE SIGNAL GENERATOR Filed July 23, 1959 14 Sheets-Sheet 1 FIG/0 F/GJ FIG. l5

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BY 9%1 fwn ATTORNEY CODER '-P' OUDER CODER CODER CODER CODER CODER P faz coDER V GODER coDER L I I fig AunIo sIGNAL J5? soURcE 14 Sheets-Sheet 2 CODER @iT-'I f W. S. DRUZ 7 9 0 5 6 uw nw Nm 6 may 6 7 Z Hm l w 7 7 M J w f... M L M w m m U u. M .IL U

I.N L L L E F LN .N NE M MHK MNE AEFL A AMW AME A A AMT M AMT ANH AE IL MHT M MPA MPA MPA M MPA MPA M M MPA R MPA MPA MPA MOM R ROG ROG MOG M MOG MOG M M MOG 0 MOG MOG MOG N nNv M nNv N N N N N N N N N N N V v Bl-'STABLE MULTI- VIBRATOR CODE SIGNAL GENERATOR .9:1 BI-STABLE Mo osTAIaILE MULTI- vI aRAToR MULTI- vI BRATOR BI-sTABLE MULTI- VIBRATOR IBI-STABLE MULTI- vIBRATo BI-sTABLE MULTI- VIBRATOR MULTI- vI BRATOR f VIBRATOR f6 BI-STABLE MULTI- VIBRATOR AELE U LTI- RA MONO- ST M VIB BII BLOCKING 82| BLOCKING T OSCILLATOR Bil BLOCKING 81| BLOCKING 8:1 BLOCKING 81| BLOCKING osGILLAToR osGILLATOH oscILLATo IaLocKING osoILLAToR OSCILLATOR 81| BLOC KING OSCILLATOR 3f oscILLAToR lso Jan. 15, 1963 Filed July 25, 1959 SYNC. SIGNAL GENERATOR GATE TIMING PULSE GENERATOR IL@ NORMALLY- CLOS ED Jan. 15, 1961" W. S. DRUZ CODE SIGNAL GENERATOR Filed July 23, 1959 Mono-STABLE 127 U WAVE BLOCKIN OSCILLATOR CONTROL CIRCUIT CONTROL BL I CIRCUIT OSCILLATOR CONTROL CIRCUIT BLOCKI G OSCILLATOR CONTROL BLOC ING OSCILLATOR I OSCI LLATOR CONTROL CIRCUIT BLOCKING OSCILLATOR BL C G OSCILLATOR BLOCKING OSC ILLATOR BLOCKING OSCILLATOR BLOCK G OSCILLATOR CON CIRCUIT CONTROL CIRCUIT BLOCKING OSCILLATOR CONTROL CIRCUIT BI. OSCILLATOR BLOCKING CIRCUIT OSCILLATOR DELAY MONOSTA LINE MULTIVI BINARY` BINARY BINARY BINARY BINARY BINARY BINARY BINARY BINARY BINARY BINARY BINARY BINARY BINARY BINARY BINARY 14 Sheets-Sheet 3 NOR ALLY C DA NORMALLY CLCSED Jan. 15, 1963 w. s. DRUz 3,073,892

CODE SIGNAL GENERATOR Filed July 23, 1959 14 Sheets-Sheet 4 NoRMALLY- T' M'NG MuLTI- MuLTI- cLosI-:D

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VIBRATR` MULTI- 215 vIBRAToR 475 MONO-ST MULTI- VIBRATOR Jan. 15, 1963 w. s. DRuz 3,073,892

CODE SIGNAL GENERATOR Filed July 23, 1959 14 Sheets-Sheet 5 0 4&1

f 1 GENERATOR BLocKlNG OSGILLATOR 280 Tal-STABLE MuLT|v| j sw @ITG/H ING o 355 ME lHRAANTSM @3 EL? f3 GENERATOR oaf 445 al M f4 NOR MALLY- GEN ERATOR NoRMALLY- OPEN 5a 335 GATE f5 GEN ERAToR NoRMALLY- oPEN GATE 305i NQTTALY- 306 f 6 GATE G ENERA NoRg/LLY- f E 4460 41000 AT L51 NoRMALLY- f 7 OPEN GEN ERAToR G 4155 NoRMALLY- NoRMALLY- cLosEo cLosEo L50 GATE 15 5 GATE RANDOM NORMALLY- PULSE MoNosTABLE M osED GENERATOR GATE DELAY L49 UNE NoRMALLY NoRMALLY- GLosED cLosEn GATE GATE MONO-STABLE MONO-STABLE NoRMALLY- NoRMALLY- MULT'V'B-RA T|v| cLosED 459 cLosEo GATE GATE AMP. 310 MONO-STABLE 286 MoNo STABLE ggg, uLT|v|BRAToR Jan. 15, 1963 w. s. DRUz CODE SIGNAL GENERATOR 14 Sheets-Sheet '7 Filed July 23, 1959 Jan. 15, 1963 w. s. DRuz CODE SIGNAL GENERATOR 14 Sheets-Sheet 8 Filed July 25, 1959 www MEG

INTERVAL Jan. 15, 1963 Filed July 23, 1959 Fl ELO-TRACE INTERVAL FIELD-RETRACE w. s. DRUz CODE SIGNAL GENERATOR 14 Sheets-Sheet 9 Jan. 15, 1963 Filed July 25, 1959 FIG. 77

w. s. DRUz 3,073,892 com: SIGNAL GENERATOR 14 Sheets-Sheet 10 Jan. 15, 1963 Filed July 23, 1959 W. S. DRUZ CODE SIGNAL GENERATOR 14 Sheets-Sheet 11 Jan. 15, 1963 w. s. DRUZ 3,073,892

CODE SIGNAL GENERATOR Filed July 23, 1959 14 Sheets-Sheet 12 STA'-'E-DETERMINING INTERVAL-P-l llll Jan. 15, 1963 w. s. DRuz 3,073,892

CODE SIGNAL GENERATOR Filed July 25, 1959 14 Sheets-Sheet 13 *f lu-l 5E Lf-STATE Jan. 15, 1963 w, s DRUZ CODE SIGNAL GENERATOR 14 Sheets-Sheet 14 Filed July 23, 1959 mx m3 3,tl73,892 Patented Jan. 15, 1963 3,073,892 CODE SlGNAL GENERATGR Walter S. Druz, Bensenville, lill., assigner to Zenith Radio Corporation, a corporation of Delaware Filed .luiy 2.3, 1959, Ser. No. 829,103 22 Claims. (Cl. TIS-5.1)

This invention pertains to a code signal generator for producing a code signal for a secrecy communication system to establish the system in a selected one of several possible operating states as determined by the code pattern of the code signal. The invention has particular application to a distortion problem which may be encountered in a subscription television system and will be described in that environment.

The code signal may be utilized in eithera transmitter or receiver.

Secrecy systems have been proposed in which an intelligence signal, for example an audio signal, is coded by altering some characteristic thereof, such as phase, at spaced time intervals determined by a coding schedule made known only to authorized receivers. Most such systems do effect adequate coding or scrambling of the intelligence signal but the signal, as coded, may have a D C. component in addition to an A C. component. Most transmitters of conventional design are not capable of transmitting a D.C. component so that only the A.C. portion of the coded signal is radiated. When the A.C. component alone is applied to the decoding apparatus of the receiver and the output therefrom is utilized to operate a sound reproducer, distortion results. Such distortion is inevitable unless the decoder operates upon the same signal as that produced by the coder at the transmitter, and the necessary identity of signals is destroyed when the transmitter radiates less than all components of the coded intelligence signal. This identity may also be destroyed in the receiver if its coupling networks do not translate the low frequency components of the received signal.

Of course, it is theoretically possible to employ a perfect, carefully designed, D C. modulator in a transmitter, such as in a frequency modulated audio transmitter, that has a high degree of stability. Moreover, la perfect frequency detector may be used at the receiver to reproduce the D C. component. If the circuits are not absolutely stable in operation, however, objectionable frequency drift results. As a consequence, it is impractical to transmit and reproduce a D.C. component of a coded intelligence signal in this manner.

One arrangement for overcoming this problem is disclosed and claimed in copending application Serial No. 513,757, iiled June 7, 1955, and issued February 3, 1959, as Patent 2,872,507, in the name of Walter S. Druz, and assigned to the present assignee. There a system is suggested for transmitting and reproducing the D.C. component as well as the A.C. component of an audio signal which has been coded by inverting the phase thereof from time to time in accordance with a code schedule. The Druz arrangement avoids the distortion otherwise introduced during the decoding process when the D C. component is not conveyed. Briefly, the D.C. compoent is amplitude modulated on a sub-carrier at the transmitter, preferably in a suppressed carrier modulator, and then both the A.C. component and the D.C. modulated subcarrier are frequency modulated on a main carrier for transmission to a receiver. VPIlle main carrier wave is rst demodulated at the receiver to recover the AC. component and the D.C. modulated sub-carrier and subsequently the D C. component is derived by means of a second demodulator such as a synchronous detector. The A.C. and D C. components are then both employed in the decoding process to develop a signal which corresponds to the original uncoded audio signal.

Copending application Serial No. 513,868, also tiled June 7, 1955, and issued October 27, 1959, as Patent 2,910,527, inthe name of Adrian I. De Vries, and assigned to the present assignee, discloses another arrangement for translating a D.C. or very low frequency component of an intelligence signal to preclude distortion. In short, the receiver of the De Vries arrangement utilizes the same type transmission as the Druz system-namely, a carrier which is modulated by a complex wave form comprising the A.C. component of the coded audio and a sub-carrier modulated in accordance with the D.C. component of the coded audio. The complex wave form is phase inverted in synchronism with phase changes at the transmitter to produce a composite signal including the sub-carrier with certain amplitude peaks of the sub-carrier representing the uncoded audio signal free of distortion. Those peaks are sampled to develop an output signal containing the sampled portions and by shaping it in a low pass iilter the original uncoded signal is simulated.

While the systems of both Druz and De Vries do eliminate the distortion otherwise present when the D.C. coniponent of the coded intelligence is not reproduced in the receiver, such systems do exhibit the obvious disadvantage that special circuitry is required at each receiver. In accordance with the present invention such distortion is eliminated by establishing a code schedule, namely, the time intervals during which phase inversion of the audio occurs, such that the D.C. component resulting in the coded audio is made as small as possible so that it is of negligible eifect.

Accordingly, it is an object of this invention to provide a novel code signal generator for producing a code signal which when utilized to code an intelligence signal results in a coded intelligence signal having a minimum D.C.

component.

It is another object of the present invention to provide a code signal generator for achieving the results of either of the previous Druz or De Vries arrangements without requiring additional special equipment at each receiver as is the case in those prior systems.

It is a further object of the invention to provide a code signal generator for producing a code signal which may be utilized to code an intelligence signal to procude a coded intelligence signal which is relatively free of distortion.

It is another object to provide a code generator for effectively selecting the manner in which an intelligence signal is coded to develop a coded intelligence signal exhibiting a characteristic having a value closest to a predetermined reference value, where the value of the characteristic is different when the intelligence signal is coded diiierently.

It is still another object of the invention to provide an arrangement for selecting from several dierently coded intelligence signals, each of which has been coded in accordance with a respective one of several operating states and each of which contains a common signal oomponent, such as a D C. component, having a predetermined magnitude, the particular coded intelligence signal which contains the common signal component having a magnitude closest to a predetermined reference magnitude but of opposite polarity with respect thereto.

An additional object is to provide a kmethod for producing from a given time segment of an uncoded intelligence signal a coded intelligence signal coded in accordance with a selected one of several possible operating states and containing a minimum D.C. component.

A code signal generator, constructed in accordance with the invention, produces a code signal for a secrecy communication system to establish the system in a selected one of several possible operating states as determined by the code pattern of the code signal. The generator comprises a source of intelligence signal and means for developing several different control signals each of which represents an assigned one of the several operating states of the system. Means are provided for simultaneously utilizing the control signals to cozde the intelligence signal to develop several differently coded intelligence signals each of which is coded in accordance with a respective one of the operating states and each of which contains a corresponding characteristic or signal component having a unique predetermined magnitude-polarity condition. The Vgenerator has means for determining which ofthe coded intelligence signals contains the common signal component having a magnitude closest to a predetermined reference magnitude but of opposite polarity with respect thereto to provide a control effect represening the selected coded intelligence signal. VFinally, the generator includes means forutilizing fthe control effect to develop a code signal having a code pattern representing the operating state according to which Vthe selected coded intelligence signal is coded.

The features of this invention which are believed to be newV are set forth with particularity in the appended claims.` The invention, together with further objects and advantages thereof, may best be understood, however, by reference to the following description taken in conjunc- 'tion with the accompanying drawings, in which:

A FIGURE 1 is an idealized graphical representation of a series of wave forms .illustrating` the penomenon which 'gives rise to distortion in a particular type of audio coding system;

FIGURES 2-6 areV schematic diagrams collectively Aillustrating an entire subscription television transmitter including a code signal generator constructed in accord- 'ance with the invention;

Vof a portion of FIGURE 3;

FIGURE 9 is a schematic representation of a subscription television receiver adapted to utilize the coded signal developed in the transmitter of FIGURES 2-6;

FIGURESlO-lS are idealized graphical representations of various wave forms helpful in explaining the :operation of the transmitter and receiver of FIGURES 2-9; and,

FIGURES 16 is a layout diagram showing the manner Yin which FIGURES l15 should be laid out to maintain the proper phase relationship between the various curves.

Before Vconsidering the structural and operational de- -tails of the illustrated embodiment of the invention, it .might prove helpful to discuss the D.C. component which arises from a particular manner of coding an audio signal and the distortion which this component may introduce into the decoded audio signal. Y

` In accordance with one effective audio scrambling technique, the phase of the audio signal is inverted from timeV totime as determined by amplitude changes of a control signal of square wave shape. The control signal may b e t. phase modulated about a mean frequency to increase the security of the system. Phase modulation of the periodi- -cally varying square wave is achieved by interrupting or disrupting the periodic pattern from time to time during spacedstate-determining intervals in accordance with a Vcode schedule so that the phase of the control signal is changed froml one to another of the intervening time in- .tervalstas between several possible operating states or phase-conditions.A

In a known subscription televisionsystem, this type of y control signal isv developed by employing a cyclic counting'mechanism, comprising an 8:1 blocking oscillator and a -bi-stable'multivibraton which is actuated in response to 'line-drive pulses to develop a square wave control signal havingamplitude changesiafterY each series'of eight line-trace intervals and thus exhibiting a frequency of approximately 1000 cycles per second. During a portion of each field-retrace interval, which may be termed a state-determining interval, a combination of randomly sequenced code signalrbursts, individually having a predetermined identifying frequency, is developed, and by means of an adjustable switching mechanism, these various bursts are segregated from one another and utilized to supply actuating pulses to selected input circuits of the 8:1 blocking oscillator and the bi-stable multivibrator in the counting `mechanism. Theapplication pattern of these actuating pulses is determined by the adjustment of the switching mechanism and this is made known only to authorized subscribers.

With this arrangement, the pulses selectively trigger the blocking oscillator and multivibrator in a prescribed randomor irregular sequence determined by the code bursts to disrupt or interrupt the normal cyclic actuation of the counting mechanismduring the state-determining or fieldretrace intervals in accordance with a secret code schedule vand rephase the square wave control signal developed thereby to a'selected one of several, specifically sixteen, possible operating states or `phase conditions. Consequently, the control signal produced in the counting mechanism exhibits periodic amplitude variations during the field-trace intervals, due to the cyclic operation of the counting mechanism, and random variations during the state-determining or field-retrace intervals, due to the action of the code bursts.

It has been found experimentally that phase inversion of the audio signal at the rate of approximately 2,000 times per second, which is realized by the utilization of such a phase modulated control signal (there being two phase inversions per cycle), achievesivery effective sound coding in that intelligibility is substantially completely destroyed. vHowever, it has also been found that when a control signal of square wave form having a frequency in the audio spectrum is employedto determine the times of phase inversion, the coded audio signal has a`D.C. component which leads `to undesirable distortion in the decoded or unscrambled audio signal reproduced by sub-- scriber receivers.

Consider, for example, the sine wave of curve Ain FIGURE 1 and assume it to represent an audio signal having a frequency of 1,000 cycles per second. If that signal is subjected toyphase inversions determined by the: amplitude excursions of a v1,000 cycle square wave con trol signal and it should happen that the inversions occur iat the zero points in each cycle, Where the wave form .crosses the zero reference axis, the coded audio signal of Vcurve B results. This signal is similar to that developed Iby a full wave rectifier and has an,A.C. component Z0 as. well as a D.C.V or unidirectional component represented, by the dashed construction line 21. i

, When the coded signal of curve B is transmitted by means of conventional transmitter equipment, the D.C.. component may be lost or it may be lost through the use of A.C. coupling networks in the receiver, leaving in either event only the A.C. component of curve C for' application tothe decoding circuit. It will be noted that the zero signal points in curve B are points of considerable negative potential in curve C. When the signal of curve 'Cj is subjected to a decoding function at the receiver, it

undergoes phase vinversions complementary to those to which the audio signal has been subjected at the transmitter. Each time a phase inversion takes place, the signal is at a level other than zerorpotential and the distorted wave form of curve D obtains. Such distortion gives rise to an objectionable ping in the reproduced audio and is attributable to the fact that D.C. component 21 of the coded signal of curve B has 'not beensuccessfully translated and employed inreconstituting the inf telligence in uncoded form. Of course, any variationin frequency between that of the instantaneous audio signal and the control signal also produces ping of`varying degrees; in fact, whenever' a phase inversion'occurs at a` acreage time other than those corresponding to the peaks in the audio signal cycle, distortion results.

It will be appreciated that if the operating state of the previously described subscription television system is adjusted during each field-retrace interval so that the phase condition of the square wave is such that the amplitude excursions occurring throughout the immediately succeeding eld-trace interval occur at times when the audio signal is passing through a peak or at least at an amplitude very close to a peak, considerably less ping is generated than that which is developed when the phase condition of the square wave is determined strictly at random during each field-retrace interval. The present invention achieves the result of actually programming the code schedule so that the transmitted coded audio signal contains a minimum D.C. or ping component.

Turning now to the structural description of FIGURES 2-5, a transmitter embodying the invention comprises a conventional synchronizing-signal generator 24 (FIGURE 2) which has one output circuit 1600 connected to a mono-stable multivibrator 25 to supply vertical or elddrive pulses thereto and another output circuit 2060 connected to one input circuit of a normally-closed gate circuit 26 to provide line-drive pulses thereto. The output terminals of multivibrator 25 are connected to a monostable multivibrator 27 which in turn is connected to another input circuit of gate 26. The output terminals of gate 26 are connected to a timing pulse generator 29 which may take the form of a well known ring counter in that it may have a multiplicity of stable operating conditions and may be advanced from one condition to the next in response to successively applied pulses. Specifically, generator 29 has eight stable operating conditions. A typical ring counter of this type is shown and described in detail on page 24 of High-Speed Computing Devices by the stati. of Engineering Research Associates, Inc., and published by McGraw-Hill Book Company, Inc., in 1950. The ring counter disclosed in that publication has a multiplicity of intercoupled electron-discharge paths to form a corresponding multiplicity of operating stages and opcrates in response to applied actuating pulses for rendering these paths conductive one at a time in a predetermined sequence.

Generator 29 has a series of output conductors 31-38 connected to the reset inputs of respective ones of a series of 8:1 blocking oscillators 41-48. Each of conductors 31-38 is connected to the output of an assigned one of the stages in the ring counter, constituting the timing pulse generator 29, t0 receive a pulse when the assigned stage is triggered. Each of blocking oscillators 41148 has its counting input circuit connected to synchronizing-signal generator 24 to receive line-drive pulses therefrom. The output terminals of blocking oscillators 41-43 are connected respectively to the input terminals of a series of bi-stable multivibrators 51-58, each of Which multivibrators also has an input circuit connected to the output of a mono-stable multivibrator 59 which has its input terminals connected to the output circuit of multivibrator 25. Each of multivibrators 5158 may be of conventional construction and may contain a pair of crosscoupled triodes.

Each of multivibrators 51-58 has two output circuits, individually connected to the anode of an assigned one of the two triodes, to provide output signals of opposed phases. One output circuit of multivibrator 51 is connected to the input circuit of a normally-open gate 61 and the other output circuit is connected to a normally-open gate 62. Similarly, the two output circuits of multivibrator 52 are connected respectively to the input circuits of normally-open gate circuits 63 and 64, the outputs of multivibrator 53 are connected to the inputs of normally-open gates 65 and 66, the output circuits of multivibrator 54 are connected to normally-open gate circuits 67 and 63, the output terminals of multivibrator 55 are connected to the input terminals of normally-open gates 69 and 70, the

outputs of multivibrator 56 are connected to the input terminals of normally-open gate circuits 71 and 72, the two output circuits of multivibrator 57 are connected to the input circuits of a pair of normally-open gates 73 and 74, and the two output circuits of multivibrator 58 are connected respectively to the inputs of normally-open gate circuits 75 and 76. For a reason to become apparent, the output leads from multivibrators 57 and 58 are reversed or transposed at the anodes of their respective triodes, with respect to the output connections from multivibrators 51-56 inclusive.

A mono-stable multivibrator 77 is connected to synchronizing-signal generator 24 to receive field-drive pulses therefrom and the output from multivibrator 77 is connected to a mono-stable multivibrator 78 which in turn is connected to a separate input circuit of each of normally-open gate circuits 61-76. The output terminals of gate circuits `@1 76 are connected to respective ones of a series of coders 71, 79, 72, 80, 73, 81, 74, 82, 75, 33, 76, 84, 77, 85, 73, 86, each of these coders having another input circuit connected to an audio signal source 37 which may comprise a microphone and audio amplifier in order to pick up and amplify the sound nformation accompanying the video information of a telecast.

Each of coders 71-36 may be similar to that which is disclosed in the copending Druz application, Serial No. 513,757, and may comprise a phase splitter and an electronic selector switch. The phase splitter supplies an audio signal from source 87 to the electronic selector switch in push-pull relationship, namely, with two different phases 180 apart, `and a square wave control signal is applied to the selector, as explained hereinafter, to select certain portions of each of the two signals from the phase splitter. With this arrangement, the phase of the audio signal is eiectively inverted each time the selector is actuated, and this occurs each time there is an amplitude variation of the control signal.

The output circuits of coders 71-86 are connected to respective ones of a series of control circuits 91-106 (FIGURE 3) which in turn are connected to respective ones of a series of blocking oscillators 111-126. Synchronizing-signal generator 24 is connected to a monostable multivibrator 127 to supply field-drive pulses thereto, and the output of multivibrator 127 is connected to the input of a mono-stable multivibrator 128, the output circuit 4099 of which in turn is connected to a triangular wave generator 129 and also through a delay line 130 to a mono-stable multivibrator 131. The output of generator 129 is connected to an input circuit of each of control circuits S11-106, and the output of mono-stable multivibrator 131 is also connected to separate input circuits of each of control circuits 91-106.

A pair of output terminals of each of the control circuits are connected across a storage condenser 133 which iii turn is connected across the input of a D.C. buffer amplier 134. A pair of input terminals of a sampler 135 is connected to buffer amplifier 134, and another pair of input terminals of sampler 135 are connected to the output of a mono-stable multivibrator 136, the input circuit of which is connected to the output of multivibrator 123. The output of sampler 135 is connected to a D.C. buffer amplifier 137 by means including a shuntconnected storage condenser 138, and the output of the buffer amplier is connected to a separate input circuit 0f each of control circuits 91-106 by way of a shunt-connected load resistor 139.

Blocking oscillators 111-126 are coupled by means of series-connected diodes 141-156, respectively, to a common input circuit of triangular wave generator 129. Oscillators 111-126 are also connected to the input circuits of respective ones of a series of binary circuits 1161-176 which are connected together to form a conventional open ring such that only one of the binaries is actuated at a time from its instantaneous operating condition to its other condition. In this way, if pulses areapplied to two or more of the binaries simultaneously, only one of them will be triggeredto its opposite condition. The output terminals of binaries 161-176 are connected to respective ones of a series of normallyclosed gate circuits 181-196, each of these gates also having a separate input circuit connected to the output terminals of delay line 130. The output circuits of normally-closed gates 181-196 are connected respectively to a series of mono-stable multivibrators 201-216 (FIG- URE 4) and also to separate input circuits of respective ones of control circuits 91-106.

lD igressing for the moment to FIGURE 8, a typical one fof control circuits 91-106 will be described in detail. For illustrative purposes, it will be assumed that control circuit 95 is shown. Portions of FIGURE 3 have `also been includedy in FIGURE 8 to illustrate the manner in which control circuit 95 operates. The output of coder 75 is coupled through a series-connected resistor 217 and a shunt-connected storage condenser 218 to the input circuit of a D.C. amplifier 219 comprising an even number of amplifying. stages so that no phase reversal is introduced. Condenser 218 is also coupled across the input terminals of a discharge switch 220 which normally presents an .open circuit tothe condenser. Another input` circuit of .switch 220 is connected to mono-stable multivibrator 131 to receive from time to time an actuating signal to close the switch and short circuit condenser 218 for discharging purposes. The output of amplifier 219 is coupled through a shunt-connected resistor 222 and a series-connected resistor 223 to the cathode 224 oa dioder225, the anode 226 of the diode Vbeing connected to ground'through a resistor 227. Cathode 224 is also connected through a series resistor 228 to the cathode 229 of ,a diode 230the anode 231 of the diode being grounded. Cathode 229 is connected through a resistor 233 to the cathode 234 of a diode 235, the anode 236 of which is connected to ground through a resistor 237. A D.C. buffer amplifier 238 connects anode 226 of diode 2275 to .anode 236 of diode 235. Cathode 234 is coupled through a series-connected condenser 240 and a shunt-connected resistor 241 to an amplifier 242 which is connected` to blocking oscillator 115.

Triangular wave generator 129 is coupled to anode 236 of diode 235 through a series-connected condenser 243. The junction of cathode 224 with resistors 223 and 228 is connected to the input circuit of a sampler 245, the output terminals of which are connected across storage condenser 133. The output of buier amplifier 137 is connected through a series resistor 247 to the junction of resistors 223 and 228, cathode 224, .and the input of sampler 245. Sampler 245 has another input circuit coupled to the output of normally-closed gate circuit h185.

Control circuit 95 of FIGURE 3 includes all of the apparatus enclosed within the correspondingly designated dashed construction line in FIGURE 8. The remaining control circuits 91-94 and 96-106 are identical in construction. Elements 133-135, 137-139, `and 129 are common to all of the control circuits. A sampler in each (of control circuits 91-94 and 9'6-106 corresponding to sampler 245 is coupled across storage condenser 133. Similarly, buffer amplier 137 is connected to similar resistors 247 in the remaining control circuits. The output of triangular wave generator 129 is connected to similar condensers 243 in each of control circuits 91-94 and Q6-,106.

Returning now to .FIGURE 4, the output terminals of multivibrators 201-208 are connected respectively to input circuits of a series of normally-closed gate circuits 251-258. Multivibrators 209-216 are also connected to separate input circuits Yof gates 251-258, respectively. Additionally, the output circuits of multivibrators 202- 209 are connected to separate inputs of an OR gate 2.59, and multivibrators 201V and 210-216 are connected to separate inputs of an GR gate 260. ORu gates 259 .and 26.0 arel constructed in well known manner, as shown and described, for example, on page 394 of Pulse and Digi,- talrCircuits, published by McGraw-Hill kBook Co., Inc., 1956, and serve as unidirectional mixers or matrices; an input signal on any one of the multiplicityA of input citrcuits of each matrix is .translated to the output circuit.

A mono-stable multivibrator 26.1 is connected to youtput circuit 1000 of sync generator 24 to derive vertical drive pulses, and the output circuit 3000 of multivibrator 261 is connected through a mono-stable multivibrator 262 to one pair of inputterminals of anormallyclosed gate circuit 263, another pair of input terminals thereof being, connected yto output circuit 2000V of sync generator 24 to receive line-drive pulses theretirom. The output circuit of gate 263 is connected to the input .terminals of a timing pulse generator 265 that may -take the Vsaine form as timing pulse generator 29 in FIGURE 2. Specically, it may consist of a conventional ring counter having eight stages resulting in eight stable operating conditions and is switched from one condition to the next in response to successively applied input pulses. Generator 265 has a series of eight output circuits 271- 278 each of which is connected to an assigned one of its eight stages to receive a pulse when the generator is triggered to the associatedcondition. Output circuits 271-278 are connected respectively to separate input circuits of normally-closed gate circuits 251-253.

The output terminals of gates 251-258 are connected in common to the reset Vinput circuit of an 8:1 blocking oscillator 279 (FIGURE 5) which has its counting input circuitconnected to output circuit 2000E of synchronizing-signal generator 24 to receive line-drive pulses. YThe output of koscillator 279' is connectedto an input circuit of a conventional bi-stable multivibrator 280' which comprises two cross-coupled triodes (not shown) and which is triggered from one to the other of its two stable operatingconditions'to `render the triodes conductive in alternation in response to successively applied pulses from oscillator 279. Multivibrator 280 has one reset input circuit designated R1 which triggers the multivibrator to a predetermined one of its two stable conditions in response to pulses applied to that input circuit and has another reset input circuit labeled R2 whichl triggers the multivibrator to its other stable operating condition in response to pulses .appliedthereto A third or common input circuit designated C is connected in common to both input circuits R1 and R2 in order to actuate the multivibrator from its instantaneous condition to its opposite condition, whatever one that may be, responsive to applied pulses over input C. The output of oscillator 279 is connected to input C of multivibrator 280.

A delay line 282 (FIGURE 5) is connected to output circuit 2000 of sync generator 24 to receive `line-drive pulses and has its output circuit connected to one input of a random pulse generator 284. A mono-stable multivibrator 285 has its input circuit connected to output circuit 3000 of multivibrator 261 (FIGURE 4) and its output terminals connected to another input circuit of random generator 284, and a mono-stable multivibrator 286 is coupled between theoutput of blocking oscillator 279 and still another input circuit of generator 284. Random pulse generator 284 may be similar in construction to apparatus disclosed and claimed kin copending application Serial No. 463,702, tiled October 2l, 1954, and issued August 2, l1960, as Patent 2,947,804, in the name of lEilers et al., and assigned to the present assignee. When energized it operates in response to pulses applied over one linput circuit (that from delay line 282) and supplies each one of those input pulses without any substantial time delay to one of a series of output circuits 287-292 selected at random. Conductors 287-292 are connected respectively to a series of normally-open gatecircuits 293-298.

i The output circuits of gates 293-298 are connected aora-,soa

respectively to stationary contacts 30M-Sola, each of which contacts is associated with a different respective one of a series of live-position rotary switches 386-301. Each of switches 381-306 has four other stationary contacts 30M-301e, 3tl2b-32e, 38317-383@ 394b-384e, 385b-305e, and 386b-306e, respectively. All of the b contacts aire connected together and then to the stationary element 387 of a single-pole single-throw switch 3818. Similarly, all of the c contacts are connected in common to a stationary contact 389 of a single-pole single-throw switch 310, and all of the a' contacts of switches 3G11- 306 are connected together and to a stationary contact 311 of a single-pole single-throw switch 312. The e contacts of switches 381-386 are joined together and connected to the common output of gates 251-258 (FIG- URE 4).

The movable contacts 3811, 392i, 3031, 364]", 385i, 3861, of switches 381-386, respectively, are connected to a series of input terminals of a switching mechanism 315 via conductors 321-326, respectively. Switching mechanism 315 has three output circuits 327-329 which are connected to the input circuits R1, C and R2, respectively, of bi-stable multivibrator 288i. Switching mechanism 315 is provided to establish any selected one of a multiplicity of possible interconnection patterns between its input circuits 321-326 and its output circuits 327-329. Movable contacts 381f-3tl6f are also connected to respective ones of a series of generators 331-336, the output circuits of which are connected in common. These generators, when energized, produce signal bursts of distinctive frequencies, generator 331 producing a signal or" frequency designated f1, generator 332 producing a signal of frequency f2, and generators 3133-336 producing bursts of frequencies fS-f respectively.

Movable contacts 30M-3861 are also individually connected to the movable contacts 44h-446:1 of a series of tive-position rotary switches 441-446, respectively. The two extreme stationary contacts of each of switches 441-446 are left open (not connected to any other circuit) whereas the middle three strationary contacts of each switch are joined together and then connected to the a contact of its associated one of switches 3811-306. Movable contacts 3811 and 441a are ganged for unicontrol action as shown by the dashed construction line therebetween. Similarly, contacts 3021 and 442:1, 383i and 443:1, 3041 and 444a, 385i and 445g, and 3869 and 446a are individually ganged, as shown by the dashed construction lines, for simultaneous operation.

A mono-stable multivibrator 448 is connected to output circuit 1068 of synchronizing-signal generator 24 to derive vertical drive pulses, and the output terminals of the multivibrator are connected through a mono-stable multivibrator 449 to one input circuit of a normally-closed gate circuit 450, the gate having another input circuit connected to output circuit 2888 of sync generator 24 to derive line-drive pulses therefrom. The output of gate 450 is connected to one input circuit of a generator 451 which when energized produces a signal burst of a distinctive frequency f7. Generator 451 also has another input circuit connected to the output circuit 4888 of multivibrator 128 (FIGURE 3), and the output of generator 451 is connected to the common output circuit of generators 331-336.

As stated before, bi-stable multivibrator 280 is conventional in construction and comprises two cross-coupled triodes. Two separate output circuits are connected to respective ones of the two anodes of the two triodes, and these output circuits are connected to a pair of normallyclosed gate circuits 453, 454. The output terminals of gates 453 and 454 are connected through respective ones of mono-stable multivibrators 455 and 456 to respective ones of a pair of normally-closed gate circuits 457, 458, which gates have further input circuits connected to output circuit 2880 of sync generator 24 to receive line-drive pulses. The output circuits of gates 457, 458 are connected to a pair of normally-closed gate circuits 459, 460, respectively, gate 459 having another input circuit connected to the output terminals of OR gate 268 and gate 460 having another input circuit connected to the output terminals of OR gate 259. The output circuit of gate 459 is connected to the movable contact 461 of single-pole single-throw switch 308 and also through a buffer amplier 462 to the movable contact 463 of single-pole singlethrow switch 312. The output circuit of gate 468 is connected to the movable contact 464 of single-pole singlethrow switch 310` and also through a buffer amplifier 465 to movable contact 463 of switch 312.

A mono-stable multivibrator 470 (FIGURE 4) is connected to output circuit 1000 of synchronizing-signal generator 24 to receive held-drive pulses, and the output of multivibrator 470 is connected to a mono-stable multivibrator 471 whose output circuit is connected to an input circuit of a normally-closed gate circuit 472 which has another input circuit connected to the common output from gates 251-258. The output terminals of gate 472 are connected to a mono-stable multivibrator 473 which is connected to the input terminals of a mono-stable multivibrator 474. Multivibrator 474 in turn is connected to separate input circuits of normally-closed gate circuits 453 and 454 (FIGURE 5), a-nd also to the input circuit of an additional mono-stable multivibrator 475. The output terminals of this multivibrator are connected to separate input circuits of each of gates 293-298 and also to the input terminals of a normally-open gate circuit 477. A mono-stable multivibrator 478 has its input circuit connected to output circuit 1080 of synchronizing-signal generator 24 to receive field-drive pulses and has its output circuit connected to a mono-stable multivibrator 479 which in turn is connected to another input circuit of gate 477. The output terminals of this gate are connected to separate input circuits of gates 453 and 454.

The circuitry described thus far constitutes the code signal generator portion of the transmitter. In FIGURE 6, a unit 485, which includes a conventional picture converting or pick-up camera tube for deriving a video signal representing an image to be televised and also includes the usual sweep systems, has separate input circuits connected to output circuits 1080 and 2000 of sync generator 24 to receive the eldand line-drive pulses and an output circuit connected to a video amplifier 486. The output terminals of this amplifier are connected to a video coder 487 which is coupled to one pair of input terminals of a mixer amplier 488. Video coder 487 may be constructed in the manner disclosed and claimed in Patent 2,758,153, issued August 7, 1956, in the name of Robert Adler, and assigned to the present assignee. As shown in the Adler patent, coder 487 may comprise a beam-deflection tube having a pair of output circuits which may be selectively introduced into the video channel as the electron beam of the tube is deflected from one to the other of two beamreceiving anodes coupled to such output circuits. One of these includes a time delay network so that the timing of the video components relative to the synchronizing components of the radiated signal varies as the beam of the deflection tube is switched between its anodes. This switching efect is accomplished by means of a beamdeection control or actuating signal applied to a pair of deflection electrodes of video coder 487. The introduction of such intermittent variations in the relative timing of the video and synchronizing components effectively codes the picture information since conventional television receivers, not equipped with suitable decoding apparatus, depend upon an invariant time relation between the video and synchronizing components of a received signal to reproduce the image intelligence represented thereby.

Mixer amplifier 488 is coupled to a direct-current inserter 489 and thence to a video carrier wave generator and modulator 490 which, in turn, is connected through a diplexer 491 to an antenna 492. Mixer amplifier 488 is also connected to synchronizing-signal generator 24 to receive the usual fieldand. line-synchronizing components and associated pedestal components. Audio signal source 87 which comprises a microphone and an audio amplifier, as described hereinbefore, is coupled through adelay line 495, which introduces a time delay of one complete fieldtrace interval for reasonswhich will lbecome apparent laten-to one input circuit of an audio coder 496, This coder may be similar in construction to each of coders 71-86 in FIGURE 2 and also responds conjointly to an audio signal Vand a phase modulated square Wave control signal to produce a coded audio signal having a series of phase inversions determined by the amplitude excursions of the control signal. The output terminals of. coder 496 are connected to an audio carrier Wave generator and modulator 497, and the modulated-carrier audio signal is supplied therefrom to another input circuit of diplexer 491.

Output circuit 2G00 of synchronizing-signal generator 24 isalso connected to the counting or common input of an 8:1 blocking oscillator 498 which, in turn, is connected to the counting or common input of a conventional bi-stable multivibrator 500. This multivibrator is preferably identical in construction to multivibrator 280 in FIGUE in that it comprises two cross-coupled triodes which are rendered conductive in alternation as the multivibrator is triggered between its two operating conditions. There are three input circuits designated R1, C and R2, pulses applied over input R1 triggering the multivibrator to a predetermined one of its operating conditions, if it is not already there, pulses applied over R2 actuating the multivibrator to the other of its two operating conditions, if it is not already there, and pulses applied over input C tripping the multivibrator from its instantaneous condition, whatever one-that maybe, to its opposite condition. The anode of one of the triodes in multivibrator 500 is connected to one input circuit of a normally-open gate circuit 501 which has anotherinput circuit connected to multivibrator 78 (FIGURE 2). The output terminals of gate 501 are connected to the defiection elements of video Acoder 487 to supply a deflectioncontrol signal thereto in order to effect actuation of coder 487 between its 'two operatingconditions and code the video components of the television signal in accordance with the code schedule represented' by the amplitude changes or excursions of the control signal. The output of gate 501 is also connected to a separate input circuit of audio coder 496 to effect actuation thereof and to phase invert the audio signal in response to each amplitude excursion of the control signal.

The common output circuit from al1 of generators 331-336 and 451 (FIGURE 5) is connected to a separate input circuit of mixer amplifier 488 in order to Vadd the signal 'bursts ydeveloped by the generators to the composite video Vsignal for concurrent radiation therewith. Generators 331-336 and 451 are also connected to the input circuit of a device 503 which contains a series of six filter and rectier units each tuned to an assigned one rof the six frequencies, )i1-,fg in order to segregate the signal bursts one from the other. Device 503 has six output circuits on which the segregated rectified signal bursts flff appear and are connected to respective ones of a series of six normally-closed gate circuits 504- 509. These gates also have separate input circuits connected to youtput circuit 2,000l of sync generator 24 to receive line-drive pulses and have their output circuits connected to the input terminals of a switching mechanism 510. Mechanism 510 has four output circuits 511- 514, output 511being ,connected to the reset input of blocking oscillator 498, output 512 being connected to the R1` input of multivibrator 500, output 513 being connected to the C input of the multivibrator, and output 514 being connected to lthe R2 input of multivibrator 500.

Switching mechanism 510is providedto establish any selected one of a multiplicity of possible interconnection patterns between gates S04-509 and output circuits 511- 514. For the airborne coding information to be itself coded, it must distinguish from the code information applied to blocking `oscillator 498 and multivibrator 500. A new adjustment of the switching mechanism is preferably made before each program interval and this information is conveyed only to authorized subscribers. Suitable switching mechanisms that provide adequate degrees of security against unauthorized deciphering are disclosed and claimed in, for example, copending applications Serial No. 407,192, filed February 1, 1954, and issued December 30, 1958, as Patent 2,866,961, in the name of George V. Morris; Serial No. 419,301, filed March 29, 1954, and issued August 19, 1958, as Patent 2,847,768 in the name of Jack E. Bridges; Serial No. 490,078, led February 23, 1955, and issued April 18, 1961, as Patent 2,980,901, in the name of George V. Morris et al.; and Serial No. 555,541, filed December 27, 1955, and issued September 8, 1959, as Patent 2,903,686, in the name of .lack E. Bridges, all of which are assigned to the present assignee.

The receiver of FIGURE 9 is constructed to decode especially the coded television signal produced in the transmitter of FIGURES 2-6. It comprises a radiofrequency amplifier, a first detector and an intermediatefrequency amplifier connected in cascade and all represented by one block 520. The input terminals of the radio-frequency amplifier are connected to an antenna 521 and the output terminals of the intermediate-frequency amplifier are connected to a second detector 522 which in turn is connected to the input terminals of a video amplifier 523. Amplifier 525 is connected Vto a video decoder 524 which develops a decoded video signal for application to the input electrodes of a cathoderay image reproducing device which is'included in a unit represented by block V525. Video decoder 524 may be similar to video coder 487 (FIGURE 6) at the transmitter except that it is controlled to operate in a cornplementary fashion in order to effectively compensate for variationsin the relative timing of the video and synchronizing components of the received television signal.

Second detector 522 is also coupled to a synchronizingsignal separator 527 which has output circuits connected to a field-sweep system and to a line-sweep system included in unit 525. The sweep systems are connected respectively to the fieldand line-deflection elements (not shown) associated with the image reproducer in unit S25.

Video amplifier 523 has another output circuit connected to the input terminals of a cascade arrangement of an amplifier, limiter and discrimnator detector, all represented by a single block 528. The output signal from the detector is translated through an audio amplifier 529 to the input terminals of an audio decoder 530 that may be identical in construction to coder 496 (FIGURE 6) in the transmitter so that when actuated -by a corresponding-control signal, it effects compensating phase inversions of the coded audio in time coincidence with phase inversions at the transmitter in order to effectively decode the audio signal. The output circuit of `decoder 530 is connected to the input terminals of a speaker 531.

Y Video amplifier 523 is also connected to -a series of filter and rectifier units represented by a single block 503 which is similar to element 5,03 in the transmitter with the one exception that it has an additional filter and rectifier for segregating the signal bursts of f7 frequency. Most of the remaining elements in FIGURE 9, designated with primed reference numerals, correspond to elements with similarly numbered but unprimed numerals in FIGURE 6. The only difference in FIGURE 9 is that the 8:1 blocking oscillator 498 operates in response to line-drive pulses from the line-sweep system in unit 525 whereas'blocking oscillator 498 operates in response to line-drive pulses from sync generator 24. The output of normally-open. gate circuit 581 is connected to both video decoder 524 :andyaudio decoder 530 to effect comple- 

14. A CODE SIGNAL GENERATOR FOR USE IN SUBSCRIPTION TELEVISION SYSTEM, INCLUDING A SOURCE OF LINE-SYNCHRONIZING PULSES OCCURRING DURING LINE-RETRACE INTERVALS, FOR PRODUCING A CODE SIGNAL TO ESTABLISH SAID TELEVISION SYSTEM IN A SELECTED ONE OF SEVERAL POSSIBLE OPERATING STATES AS DETERMINED BY THE CODE PATTERN OF SAID CODE SIGNAL, SAID GENERATOR COMPRISING: A SOURCE OF AUDIO SIGNAL; A PLURALITY OF COUNTING MECHANISMS; MEANS COUPLED TO SAID SOURCE OF LINE-SYNCHRONIZING PULSES FOR DIFFERENTLY CYCLICALLY ACTUATING SAID COUNTING MECHANISM TO DEVELOP SEVERAL DIFFERENT SQUARE WAVE SHAPED CONTROL SIGNALS HAVING THE SAME COMMON FREQUENCY AND EACH HAVING PERIODICALLY RECURRING AMPLITUDE VARIATIONS EXHIBITING A UNIQUE PHASE CONDITION WITH RESPECT TO THE AMPLITUDE VARIATIONS OF THE OTHER CONTROL SIGNALS TO REPRESENT AN ASSIGNED ONE OF SAID SEVERAL OPERATING STATES OF SAID SYSTEM SO THAT THE CORRESPONDING AMPLITUDE VARIATIONS OF ALL OF SAID CONTROL SIGNALS OCCUR DURING DIFFERENT LINE-RETRACE INTERVALS; MEANS FOR SIMULTANEOUSLY UTILIZING SAID CONTROL SIGNAL TO PHASE INVERT SAID AUDIO SIGNAL TO DEVELOP SEVERAL DIFFERENTLY CODED AUDIO SIGNALS EACH OF WHICH IS CODED IN ACCORDANCE WITH A RESPECTIVE ONE OF SAID OPERATING STATES, IN THAT EACH EXHIBITS PHASE INVERSIONS OCCURING DURING DIFFERENT LINERETRACE INTERVALS, AND EACH OF WHICH CONTAINS A UNIDIRECTIONAL COMPONENTS HAVING A UNIQUE PREDETERMINED MAGNITUDE-POLARITY CONDITION; MEANS FOR SELECTING THE ONE OF SAID CODED AUDIO SIGNALS IN WHICH SAID UNIDIRECTIONAL COMPONENT IS OF A MAGNITUDE CLOSEST TO A PREDETERMINED REFERENCE MAGNITUDE BUT OF OPPOSITE POLARITY WITH RESPECT THERETO AND FOR PROVIDING A CONTROL EFFECT REPRESENTING THE SELECTED CODED AUDIO SIGNAL; AND MEANS FOR UTILIZING SAID CONTROL EFFECT TO DEVELOP A CODE SIGNAL HAVING A CODE PATTERN REPRESENTING THE OPERATING STATE ACCORDING TO WHICH THE SELECTED CODED AUDIO SIGNAL IS CODED. 